This could be due to a software issue. Perhaps your software is not properly telling your hardware what to do. Check your system settings within your camera's menus. Also make sure your camera has up-to-date software. Another issue could be internally. Perhaps your camera's flash is not properly connected to the main circuitry, thus not giving it access to power. If this is the case, you will have to disassemble the entire camera frame to get to the flash. Beware that doing this carelessly could result in your camera losing some of its "proofing" capabilities.